We were allied with the Russians during the war, and we were fighting them after the war. It is an interesting turn of events. During the war, it was the enemy of my enemy is my friend. Once the common enemy of the Germans was extinguished, we could be enemies again.

Also, during World War II in China, the communists under Mao Zedong and the Nationalists under Chiang Kai Shek, whose forces had been fighting a civil war for years, agreed to a truce and focused their individual efforts to fight against the Japanese invasion and occupation. Theirs was an alliance of convenience as well, as they both decided that japan was a greater threat than each other.

After the war ended and Japan was defeated, the civil war resumed, and over the next four years, the nationalists were slowly overcome, and by 1949 the communist People's Republic of China gained control of the mainland, and the nationalists were chased to present day Taiwan.

Just as in Europe, an alliance of convenience was formed when the external threat was greater, the Nazis in Europe, the Japanese in Asia. As soon as those threats were dealt with, the old rivalries resumed in the Cold War.

To answer this question, simply think about who was allied with whom during the war and how those alliances changed after the war.

During the war, it was the US, the USSR, Great Britain, and France (and a few others) against Germany, Japan, and Italy. After the war, it was all of those countries (with the exception of a part of Germany) against the Soviet Union.

The alliance between the US and the USSR during the war was a marriage of convenience. It did not last after the war because the two sides did not trust one another. As the Cold War began, all of the other countries sided with the US in an attempt to avoid the fate of Eastern Europe (which had fallen under Soviet domination).